How many number of ports are vunerable for an attack in a computer?

By ports you could be referring to physical connection ports on a host machine, if so then all can be abused in some form to compromise or damage a machine. Physical access to a computer is often considered a 'game over' in terms of security.

IF you mean TCP/UDP connection ports (network) then any open port can offer an attack surface. It is best practice to close all unused ports leaving open on those specifically needed. Using a firewall to do this is common practice, these can be in the form of software, embedded or dedicated hardware. Closing unused ports on edge devices (routers and servers) is a standard move when setting up a network.

Be aware that any ports that are open can be subject to port redirection attacks, in this instance the use of a firewall and Access Control Lists (ACL's) can help mitigate this threat.